This invention relates to methods and systems for determining the position of a transponding platform that is part of a multi-transponding platform communication system and of users, using two-way ranging navigation. This invention also relates to a method and a system for calibrating the Global Positioning System using a two-way ranging navigation system.
Current Automatic Dependent Surveillance (ADS) technology in the form of a Global Positioning System (GPS) provides information utilizing satellite communication. The GPS, developed by the United States Department of Defense, consists of twenty-four (24) satellites that orbit the earth twice a day at an altitude of 12,000 miles. Additionally, the GPS consists of five (5) ground stations that monitor and manage the satellites. Using atomic clocks and location data, GPS satellites transmit continuous time and position information twenty-four (24) hours a day to a GPS receiver, which inputs data from three or more satellites simultaneously to determine the user""s position on the earth. By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and each satellite. The GPS receiver then uses the distance measurements from at least three satellites to arrive at a final user position or location.
The civilian GPS measurement yields an accuracy of approximately 100 meters. Additional accuracy, at a level needed for targets such as aircraft, is obtained by using a Differential GPS consisting of ground reference points. However, ground reference stations are expensive to deploy, and their effectiveness is limited to a finite radius from the reference position on the surface of the earth.
The civilian GPS system utilizes one-way ranging in which an accurate, synchronized clock is required at each station. Any synchronization error, or any error regarding the location of one of the satellites, results in an error in the determined position of the target. An aircraft, or other target, must have accurate position and velocity information and requires dedicated, precise equipment throughout the flight or mission to provide accurate position and velocity information with a high degree of integrity and reliability.
To overcome this problem, two-way ranging navigation has been proposed by co-pending U.S. application Ser. No. 08/803,936, which is incorporated herein by reference. The two-way ranging navigation system disclosed therein determines a position of a vehicle, such as an aircraft or surface vehicle, utilizing two-way ranging techniques through multiple satellites to derive independent estimates of a vehicle""s state vectors including position and velocity.
The two-way ranging navigation system includes transmitting a first signal to a vehicle from a traffic control station through a plurality of satellites. From that signal, the vehicle acquires and tracks a unique ranging code, and a message signal containing the unique code is transmitted from the vehicle. The links through multiple satellites are sequentially or simultaneously processed to provide two-way ranging with reasonably accurate estimations of state vectors. The two-way ranging navigation system processes ranging measurements to derive estimations of vehicle state vectors in response to a first signal and a message signal at a traffic controller, preferably a ground-based traffic controller employing xcex1-xcex2/EKF (Extended Kalman Factor) tracking.
Two-way ranging navigation can be used as a stand-alone positioning system for a target with higher accuracy than GPS systems. During normal communication links, positioning with two-way ranging navigation consumes little space-segment resources. However, when there is no communication between the user and the satellite, also known as xe2x80x9csilent intervalsxe2x80x9d, two-way ranging navigation positioning requires additional space-segment bandwidth resources. GPS, on the other hand, being a one-way broadcast positioning system, requires no additional space-segment resources once it is deployed.
Therefore, there is a need to obtain the highest degree of positioning accuracy without the cost of two-way ranging navigation positioning and while utilizing the free civilian GPS signals whenever possible. There is a need for precisely and quickly locating the position of a transponding platform above-ground or mobile user or vehicle on the ground or on water in a multiple transponding platform communication system.
The communication system of the present invention includes a plurality of individual communication transponding platforms. The plurality of individual transponders are each in communication with a central processing hub such that a signal processed by the central processing hub in the forward link is radiated with compensating time delays to the plurality of individual transponders. The radiated signals are then re-radiated by the plurality of individual transponders and coherently received and processed by a mobile user terminal. The return link signal path is the reverse of the forward link.
The present invention preferably uses a xe2x80x9cGeo-stationaryxe2x80x9d stratospheric platform (xe2x80x9ctransponding platformxe2x80x9d) that is suitable for microwave and other payloads similar to those used in satellite communications systems. Since the distance between the ground station and the platform is around twenty km, the delay between transmission and reception is reduced to less than 0.06% of that of a geosynchronous satellite transmission system.
The present invention provides a method and system for using two-way ranging navigation to accurately determine the range along the path from a source to a destination by measuring the time delay during round-trip communication via at least three transponding platforms for an above-ground target such as a stratospheric platform whose position needs to be determined, and via at least two transponding platforms for an on-ground target such as a mobile user whose position needs to be determined. The two-way ranging navigation measurements are used as calibration references, thereby improving the positioning accuracy of GPS without the cost of implementing Differential GPS and without the physical limitations associated with Differential GPS.
The system for calibrating GPS using two-way ranging navigation includes GPS and a two-way ranging navigation system for determining position measurements of a target. A correction factor is determined as a function of measurements taken by the two-way ranging navigation system and the GPS, and the GPS position of the transponding platform or other above-Earth target is adjusted by the correction factor. The user can use two-way ranging navigation periodically to update the correction factors. The more frequent the update, the less error there will be in the correction factor.
The method for calibrating GPS using two-way ranging navigation includes taking a two-way ranging navigation measurement, at a first predetermined time, of an above-Earth target, such as a transponding platform, or a target on the ground or water, such as a moving or stationary vehicle or user or fixture, all herein referred to collectively as xe2x80x9ctargetxe2x80x9d. A first GPS measurement of the target is also taken at the first predetermined time. A correction factor is determined as a function of the two-way ranging measurement and the GPS measurement. The GPS position is corrected by taking a second GPS measurement of the target at a second predetermined time and adjusting the measurement by the correction factor to obtain a corrected GPS position.
This method is especially advantageous for transponding platforms in wireless communication systems. Higher accuracy can be obtained, when desired, by performing more two-way ranging navigation measurements whenever, and wherever, needed.
It is an object of the present invention to precisely locate the position of a transponding platform that is part of a multiple transponding platform communication system.
It is also an object of the present invention to precisely locate a mobile user or vehicle on the ground or on water using two-way ranging navigation.
It is an additional object of the present invention to precisely locate a fixed target on the ground using two-way ranging navigation.
It is also an object of the present invention to improve the accuracy of GPS without adding unwanted cost and complexity. It is another object of the present invention to utilize two-way ranging navigation as a calibration reference for GPS. It is yet another object of the present invention to broadcast calibration information to all users within a predetermined region.
It is a further object of the present invention to maintain flexibility in the calibration method so as to allow different levels of accuracy based on different applications for navigation. It is still a further object of the present invention to provide a back up navigation system for GPS in the case of GPS failure.
The above objects and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings and appended claims.